Section of One-half of a Nave Rib, showing the cross-bracing in the Gallery,

ment to the nave and transept roof, and the particular wooden principals going straight across, has two iron form of bracing by which this is effected is the in- diagonal ribs crossing it, forming as it were a groined genious suggestion of Mr. Ordish. The roof thrust-arch, whose apex is a point in the centre of the bay and ing outwards tends to throw the columns out of the per- in a line with the roof ridge. By joining the apices of pendicular; strong iron braces are, therefore, anchored to these groins and the points in the octagon already deterthe foundation of the inner column, and carried up to the mined, we get a nearly regular dodecagon, having its top of the opposite outer column, which are thus made to opposite sides parallel and equal, and with eight sides in counteract the thrust of the roof. Another bracing, an- pairs each equal to 43′ 9′′, and the four remaining chored to the footing of the outer column, is carried up to sides coming between these pairs, each equal to 35′ 5′′ the top of the inner column, to secure it from being acted This dodecagon forms the base of the dome, which will on by the force of the wind. This vertical cross bracing thus have eight sides over the nave and transepts, and is repeated at every 100 feet, or every fourth bay, and by four sides over the corners of the aisles, equal respectively introducing horizontal diagonal bracing under the roof to the dimensions just given, and a diameter of 160 feet. flats, they are turned, as it were, into a deep horizontal Each groined rib transmits the weight on it to two girder, supported at two ends by the columns vertically columns outside the octagon, so that the dome may be braced as just described. This horizontal girder, therefore, said to rest on 16 points, its pressure on the angles of the takes the thrust of the three intervening ribs. The way octagon being nearly five times (4) as much as it is in which the bracing is introduced is very clever, and is an on the adjacent columns of the nave and transepts. admirable example of the perfect control which the By the very ingenious and novel plan of the groined simplest mechanical means, properly applied, gives us in roof ribs a dodecagon dome is made to seem to stand on dealing with enormous inasses. The bracing is all ad- an octagon; no additional columns of support but those justed by connecting screw links, on a plan very similar actually coming in the sides of the nave and transepts are to the method of joining railway carriages; by this means used, and thus an uninterrupted vista is obtained through it can be tightened at pleasure, and the position of the both, and a very beautiful architectural effect produced. columns corrected to the minutest fraction of an inch. The drawing of the rib also shows the vertical cross bracing, which will, I hope, be thus made perfectly clear to you. (See woodcut p. 47.)

OPEN OR GLASS COURTS.

You will recollect that in describing the position of the aisles and galleries, I explained to you how they enclosed six courts, three north of the nave, two of which are 250 by 87 feet, and the other 150 by 87 feet-three south of the nave, two of which are 250 by 200 feet, and the other 150 by 200 feet. These form the open or glass covered courts, and are the only portions of the building which in this particular resemble the Crystal Palace of 1851. They have only a ground-floor, and the roof, which is on the ridge and valley plan, but in spans of 50 feet, is entirely covered with glass. The roof is carried on square iron columns 50 feet apart each way, at the top of which, 50 feet above the ground, wrought-iron trellisgirders are fixed on lines running east and west. The columns and trellis-girders carry the principles of the roof, which are all of iron, on the trussed rafter plan, eight feet apart. The roofs are drained by channels in the vallies conducting the water down the hollow iron columns. The effect of these courts with their light glass roofs admitting floods of light into the building will give a pleasing variety to the interior, and afford most valuable exhibiting space. The whole of the S. W. court, with a portion of the adjoining aisles and galleries, has been allotted to France, which will have the greatest space of any foreign country.

DOMES AND GROINED RIBS.

We now come to the great domes, which, from their stupendous size, will form one of the most prominent and interesting features in the building.

I have before explained that they are situated at the intersection of the nave and transepts. Their form and position have been thus determined. The intersections of the lines of columns in the nave and transept aisles form two octagons, which, though not mathematically regular, are regular in this one respect-their opposite sides are parallel and equal, the length of the sides being alternately 85 feet and 35' 5". The points at the angles of these octagons are the chief supports of the domes. For this purpose there is a column at each angle, 2 feet in diameter, and for architectural effect, as well as for carrying the groined ribs, the object of which I shall presently explain, the lower portion of these 2-feet columus is clustered with two round and one square column of smaller dimensions.

But though the chief points of support are at the eight angles of the octagon, the dome is a dodecagon, the other four points being thus obtained. The last bay of the nave and transept, instead of having a roof resting on

You will see a drawing showing a projection of one of the groined ribs on a line at right angles to the nave, its curve forms a semicircle, to correspond with the curve of the other ribs, but its actual elevation will show a semiellipse, whose diameter, 88 feet, is the diagonal of the rectangle formed by the four supports of the groins.

in.

Each rib is two feet deep, with a web of in. plate-iron, to the edge of which is rivetted a top and bottom flange formed of angle-iron in such a way as to give the top flange an area of 10 in., and the bottom flange an area of 19 in. The principal rafter and its upright are also made of wrought iron, having a web 12 in. deep, with an equal top and bottom flange of angle iron rivetted to it so as to give it a sectional area of 20 Radial pieces of iron 8 X in., connecting the upright and principal rafter with the circular portion of the rib, are introduced every 5 feet. At the intersections the ribs are strengthened by additional plates of iron, and here, for a short distance, they assume the form of a box girder. A drawing, showing the details of these portions on a large scale is provided, and will, I hope, fully explain their construction.

The intersections of the principal rafters and semi-ellipses are connected together by a cast-iron standard, which is continued up above the ridge of the roof to a point 107 ft. from the nave floor line, this being the level of the bed on which the dome ribs rest.

The large columns at the angles of the octagon are two feet in external diameter with seven-eighths of an inch of metal, and they are raised in three lengths to a height of 95 feet, their ends being joined together by flanges and screwed nuts on the inside. To fasten the bolts, a man is lowered down inside the columns, the diameter of which is sufficient to give room for him to screw up the nuts. The columns are thus kept perfectly smooth on the outside, and appear like one casting 95 feet long. To the top of each two-feet column is bolted a cast iron stanchion 12 feet high, whose summit is therefore just 107 feet above the nave floor. On the tops of these stanchions, and resting on ornamental brackets, a gallery three feet wide is carried round the outside and inside of the drum. It will not, however, be accessible to the public, but only to men employed in opening the louvres which are here placed for ventilation. To the upper side of the gallery, and through it to the stanchions, the double wrought-iron tie-plate acting as the dome's hoop, is securely bolted. It consists of an inner plate of iron x 6"X" 6 × 1, which is connected with an outer plate 10" X 10 X, so that both these plates take the thrust of the dome. The dome ribs are bedded on the top of them, with their feet bolted through to the heads of the stanchions.

Each dome rib is an iron girder made of boiler plate and

ing line. The top flange follows the curve traced by a radius of 91' 9", the centre being a point 12' 3" beyond the centre of the dome; the bottom flange is on a curve whose radius is 90' 1", and centre 14' 0", beyond the dome's centre.

angle iron. The top and bottom flanges are nearly equal in section, the former being 198", the latter 203" 204. There is no continuous web between the two flanges, but they are joined at 8 feet intervals by two pieces of boiler plate, having a 3" wood spacing piece between. The first seven feet of each rib is vertical, and the girder is The two flanges thus come nearer each other as they here 3 feet deep. At the summit of the vertical portion, approach the apex, where they are only two feet apart. which is 114 feet above the nave floor level, is the spring-This point is 91 feet above the springing line, and the

PLATE

......

Elevation of Diagonal Ribs, supporting the Rib of Dome over Nave and Transepts.

12 ribs meet there, abutting against a strong cast-iron they approach the summit, and those in the larger tripipe, one foot in diameter, to which they are bolted. angles are slightly heavier, on account of their longer Eight wrought iron purlins between the springing and bearing. The first purlin of a large triangle is 1' 818" the apex are bolted to the ribs, and the divisions thus deep, and its T-iron flanges are 3 X 3"X". The formed are strongly cross-braced, so as to make the whole upper purlin has a continuous web plate 1018" deep, with as rigid as possible. These purlins are formed of two top and bottom flanges made of four pieces of iron, pieces of T-iron, joined together at six-feet intervals by a 2" X 2" X". These dimensions are altered in a small half-inch plate. They vary in section, decreasing as triangle, the depth of the lower purlin being 1' 818",

Plan of the top of the Groined Rib.

Section on Line ab in elevation, showing junction of Diagonal Ribs at crown of Arch.

The crown of the dome, for about thirty-two feet down, has an ornamental zinc covering, but the whole of the remainder is glazed. From the apex rises the finial to a height of fifty feet, resting on a concave base, which, being

Top Plate to receive Column.

INTERSECTION

LINE OF

Section on Line c d in elevation, showing junction of Top of
Diagonal Ribs.

prettily ornamented with cast-iron brackets, windows, and
mouldings, is terminated by a globe surrounded by three
great circles intersecting each other, from the top of which
rises a gilded pinnacle.

Section of Seat of Diagonal Rib, separated from Square Ribs. exterior diameter, the dome of St. Peter's is 157 feet, and that of St. Paul's 112 feet. But, although our domes are in themselves larger in every way than any yet constructed, they will not rise to so great a height above the ground as either of those with which I have compared them. The Exhibition domes spring from a height of 114 feet, which makes the top of the finial 260 feet above the ground, while the cross on St. Peter's is 434 feet, and that of St. Paul's 340 feet above the pavement.

DOME SCAFFOLDS.

The scaffolds for the construction of these domes are on a greater scale than anything of the kind ever executed. They are literally forests of timber, occupying nearly the whole interior space of the domes, cross-braced and bolted together in every possible way, so as to give them sufficient strength, for they will have to bear the weight of the whole of the iron in the domes, 120 tons in each.

The scaffold is carried up in eight different stages, between which are horizontal beams. The central portion is a square of 24 feet, rising to a height of 200 feet. As this ascends each stage is cross-loraced vertically. From the centre radiates a scaffold into each triangle of the dome, to which it is in shape si nilar, though not quite so large. These radiating scaffolds have independent vertical bracing, while at each stage they are cross-braced horizontally, and connected with the central scaffold as well as with each other. The main timbers in the scaffold are from 14" to 12" square. while the cross-bracing is, on an average, 12" x 6". This work was put up by Mr. Clemence, the contractor's clerk of works, and must be considered a chef-d'œuvre in scaffolding; it is of immense strength, and so skilfully constructed, that very little of the timber in it has been spoiled by cutting, so that when taken down, every particle of wood used, amounting to 40,672 cubic feet in each scaffold, will be as available for any other work as if it had just come from the builder's yard.

Section on Line e ƒ in elevation, showing Section of Seat of Diagonal which will show you all the details of construction.

Among the drawings you will see a plan of the scaffold,

Rib on corner column of Dome.

Having now gone through the various portions of the permanent buildings, I shall proceed to a description of the annexes or temporary buildings adjoining the Ex-ing; any person of ordinary intelligence, able to drive hibition.